I need an alien race to serve as their enemies, very difficult enemies, but have few ideas so far. They should be distinctive, possibly their complete opposite.

The Krys tend to be slow, defensive, liking to have the upper hand - dominating the situation with big, powerful ships if possible. Their enemy should be in turn fast, aggressive, chaotically attacking and baffling the commanders; with large numbers of smaller ships, that could quickly switch from feigning a target to hitting the real one. They probably have a slightly lower technology level (or less resources). Occasional suicidal tendencies are fine, but I don't want to overdo it.

Strategically, the Krylastys are of course stronger, but regularly find themselves at a tactical disadvantage... these guys jump out of nowhere, destroy or damage important targets, then vanish again.

Warfare terms aside, what I am really missing is a personal touch on them, or any idea of the nature of the conflict. And I am open to your suggestions.

The united fleet of vanquished foes. Their tech level differs, but some of their greatest ships are flagships of devastating power. Unfortunately they cannot replace said ships if they are lost, so these ships travel the uncharted sectors, hiding whenever possible, and are only committed to extremely important battles.

Meanwhile they deploy ragtag fleets of smaller craft to attack the Krylastys strongholds. Guerilla warfare is their only option and they have to operate mines in dangerous asteroid fields, or raid supply depots and cargo vessels, if they want to stay operational. Planetary facilities are out of the question as the Krylastys are way too powerful if they can focus their forces on a target.

I've let this rest for a while, since none of the ideas went quite with what I wanted (though they were both good - AG's vanquished foes in particular sounds worth doing).

But I want still something different.

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notes for later on, sketch of the enemies forming: - predatory nature, as opposed to herbivores - technologically somewhat below the Krylastys - ?they are actually preying on the high-quality materials the Krys build their structures from - special: ?claw-like weaponry - separating material on the molecular level - either some sort of beam or (sigh) monofilament wire - would require some exposure to the weapon to work - coordinated attacks, synchronized targeting - they cut apart the finest pieces of equipment and drag it away - the Krys can usually defend themselves; but not their outer structures - a typical attack means few casualties, but large damage - most attacks are in freshly colonized systems; a few established colonies were attacked as well

Originally cartilagenous fish very similar to Earth's sharks, the Selachim were uplifted some ages ago and have since existed in a technological state of limbo. They adapted well to higher intelligence, and their existance in a three-dimensional world made them natural pilots. The race that uplifted the Selachim no longer exists, as they drastically underestimated the aggressive and predatory nature of the creatures they had raised to intelligence. The Selachim turned on their creators, and what they did not destroy, they devoured.

In the centuries that followed, the Selachim spread through the galaxy, laying vehement claim to any watery worlds they found, but largely ignoring the rest. Their technology hasnt changed much since they slew their creators, and as they lack a fully industrial infrastructure they are forced to hunt down non-Selachim ships to render them down for material components and manufactured goods.